With reference to FIG. 1, a ducted fan gas turbine engine generally indicated at 10 has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high-pressure compressor 14, combustion equipment 15, a high-pressure turbine 16, and intermediate pressure turbine 17, a low-pressure turbine 18 and a core engine exhaust nozzle 19. A nacelle 21 generally surrounds the engine 10 and defines the intake 11, a bypass duct 22 and a bypass exhaust nozzle 23.
The gas turbine engine 10 works in a conventional manner so that air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first air flow A into the intermediate pressure compressor 14 and a second air flow B which passes through the bypass duct 22 to provide propulsive thrust. The intermediate pressure compressor 13 compresses the air flow A directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high-pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted.
The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines 16, 17, 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate pressure compressors 14, 13 and the fan 12 by suitable interconnecting shafts.
The fuel flow supplied to gas turbine engines can contain water. During operation under low ambient temperature operating conditions if the fuel is at a temperature below 0° C., the water will freeze. In these circumstances the fuel will normally enter the engine containing dispersed ice particles in suspension in the fuel. These ice particles present a problem for the fuel system in that they can block small passages and cause fine clearance valves to malfunction.
This problem is normally overcome by heating the fuel, using heat from the engine oil, via an fuel-oil heat exchanger. A typical arrangement of such a fuel-oil heat exchanger is shown in FIG. 2, A fuel-oil heat exchanger 30 is arranged in the fuel flow path 60 from the fuel tank (not shown) to the burners 71 of the engine. Fuel passing through the fuel-oil heat exchanger 30 is heated by an oil circuit 80 which supplies hot oil from the engine through surface coolers 81 (or via oil bypass valve 82) to the heat exchanger 30. The heated fuel subsequently passes to the low pressure filter 65 to the hydro-mechanical unit 68 which performs fuel management functions in response to pilot demand for thrust and controls turbine case cooling (TCC) and the variable stator vane actuator (VSVA). The fuel then passes through flow meter 69 and high pressure filter 70 to the burners 71.
However, if there is ice accumulation in the fuel pipe work supplying the engine, this ice accumulation can break away or “shed” and result in a large quantity of ice arriving at the engine over a short period of time. Such quantities of ice can overwhelm and block the fuel-oil heat exchanger and as a result interrupt the fuel supply causing the fuel system to malfunction and the engine to loose power.
Prior art systems, for example as shown in FIG. 2, address this problem by providing a bypass valve 64 which provides an alternative route for fuel to reach the engine. If the heat exchanger 30 becomes blocked with ice a pressure operated bypass valve 64 opens to allow fuel to bypass the blocked heat exchanger and still be supplied to the downstream components 65-71 described above to enable the engine to continue to operate. The heat exchanger fuel side inlet is designed to have sufficient volume to accommodate expected transient quantities of ice, such that when the bypass opens only fuel with the normal (steady state) level of ice concentration will pass to the downstream fuel system components.
However, when the fuel side of the heat exchanger 30 is blocked and the bypass valve 64 is open, the fuel flow to the downstream components is no longer heated. Such fuel will still contain ice and the continued operation of the engine in this state therefore relies on the ice tolerance of the downstream components. In many cases, these components (in particular units such as filters) will only have a finite capability to deal with the ice content of the fuel. This means that it is important that the ice collected in the heat exchanger inlet melts quickly so that it can properly pass fuel and provide heated fuel with no ice. For example, the low pressure filter 65 may also need to be provided with a bypass valve 66 which operates if the pressure difference across the filter as measured by the pressure sensor 67 becomes too great due to ice accumulation.
Another problem with this arrangement is that it relies on the transiently high level of ice in the fuel to occur over a short period of time, such that all of this ice is captured in the heat exchanger inlet before the bypass is opened, otherwise fuel could be delivered to the downstream components which still contains high concentrations of ice which would cause malfunction of those components.
Aspects of the present invention seek to address or ameliorate the above problems.